The invention relates to a high definition color television transmission system in which, from a wide band luminance information signal having lower, middle and upper frequency components, there are formed a first luminance portion containing the lower frequency components of said luminance information signal and a second luminance portion containing the higher frequency components of said luminance information signal, the first luminance portion being transmitted by way of a first transmission path and the said second luminance portion being frequency shifted such that the resulting frequency shifted second luminance portion lies within the bandwidth of a second transmission path which bandwidth is substantially equal to that of the first transmission path, the frequency shifted second luminance portion being transmitted by way of the second transmission path. The invention also relates to television transmission equipment and television reception equipment for use with such a transmission system.
Such a transmission system has been disclosed in the book "Video Tape Recording" by Julian L. Bernstein, published by Johm F. Rider Publisher, Inc. New York, where, in chapter 6, at page 94, it proposes to divide a television signal into four parts of equal bandwidth and to frequency shift (heterodyne) the three higher frequency parts into the same frequency band as that occupied by the lowest part. The television signal with which this disclosure is concerned is a narrow band signal of 4 MHz bandwidth and is divided and frequency shifted so that it can be recorded on four separate tracks of a tape recorder. It would appear that the television signal in question would be for monochrome display while from the tape speed, the four tracks would be recorded linearly along the tape. Since the publication of this book, considerable improvements have been made in video recording through video tape recorders for analog signals still have a restriction on the bandwidth (a few MHz's) that can be recorded.
The color television signal currently transmitted by the broadcast authorities in the United Kingdom, uses the PAL system with the transmissions taking place in the U-H.F. bands IV and V. In such transmissions the luminance information has a bandwidth of 5.5 MHz with a color subcarrier located at a point nominally 4.43 MHz above the carrier frequency, the color subcarrier being modulated by the color information. Substantially all of the currently manufactured color television receivers do not fully utilize all the luminance information transmitted. The luminance information above 3.5 MHz in a receiver is reduced in magnitude to simplify the operation of decoding the color information and with such simplified decoding, cross-luminance interference would be observed on the display without such reduction. Considerable interest has however been shown in increasing the quality of the display and this can be done by using the whole of the luminance information transmitted.
Various suggestions have been made to further increase the bandwidth, and hence the quality and definition, of transmitted color television signals. One such suggestion contemplates the transmission of a single sideband luminance modulated signal of 10 MHz bandwidth with the modulated color subcarrier being located in the other sideband. Such a transmission could not readily be currently transmitted in the U.H.F. bands IV or V as the required bandwidth would overlap into an adjacent channel. In addition, such a transmission could not be received by television receivers currently manufactured and would thus be only receivable by specially constructed receivers. A further suggestion proposed by NHK (Japan Broadcasting Corporation) in their NHK Laboratories Note, Ser. No. 239, August 1979 contemplates the separate transmission of luminance and color information in separate channels and suffers from similar objections to those above. Yet a further suggestion comes from the BBC (British Broadcasting Corporation) in their Engineering Press Release "The BBC moves towards better quality pictures", dated Feb. 23, 1982, where for wide-band transmissions such as those via satellite or optical fiber cable, it is suggested that the high frequency luminance information above 3.5 MHz be filtered off and this higher frequency information frequency shifted upwards to a higher band (8 MHz upwards) and transmitted together with the original low frequency information and chrominance signals. Such a system cannot be used with current channel spacing in the U.H.F. bands IV and V and, although the low frequency luminance information and chrominance signals could be used to provide a reasonable display in most currently manufactured color television receivers, any receiver designed to utilize the full 5.5 MHz luminance bandwidth would produce a display that is impaired by the signal of this suggestion.